The molecular basis for the hereditary type I protein S (PS) deficiency was investigated. DNA sequence analysis of 12 patients with PS deficiency in Japan identified four point mutations and three of them were novel. Nonsense mutations found in two unrelated patients resulted in termination of the PS polypeptide chains at Gln 238 and Lys 392, respectively. Two novel missense mutations were also found in two other patients substituting Asp 202 for Asn and Leu 298 for Pro, respectively. Comparison of the PS amino acid sequences from several mammalians indicated that Asp 202 and Leu 298 were preserved and thus appeared to be responsible for the pathogenesis of PS deficiency.